DNA ligase is an enzyme having an activity of linking between the 3′-OH group of a DNA strand and the 5′-phosphate group of another DNA strand through phosphodiester bond and participates in DNA replication and repair in vivo. In recent years, Ligase Chain Reaction (LCR) method has been developed and used as a novel gene amplification technique. The LCR method is a method of amplifying or detecting target genes by way of thermal cycling reaction using thermostable DNA ligase. For enhancing the efficiency of the LCR method, additional thermostable ligase has been explored and has been available commercially. DNA ligase derived from a hyperthermophilic archaebacterium has been found very recently (see the press release dated Sep. 10, 2003, as titled “Success in Development of the Most Thermostable Enzyme (DNA Ligase) in the World”, on website of National Institute of Advanced Industrial Science and Technology at http://www.aist.go.jp/aist_j/press_release/pr2003/pr200 30910/pr20030910.html). However, these thermostable DNA ligases exhibit exceedingly low DNA-binding ability. On the other hand, DNA ligase derived from a phage is known as an enzyme having high DNA-binding ability. However, this DNA ligase has poor thermostability and therefore, is not suitable for the LCR method. Accordingly, DNA ligase with excellent thermostability and DNA-binding ability and reactivity capable of efficiently performing the LCR method at a sufficient reaction rate has not been found yet.